Hitherto, various curable liquid compositions which are cured to give rubbery materials have been developed. Among them, a curing system having good depth curability has been developed, which system is prepared by cross-linking a polyorganosiloxane having on the average two or more vinyl groups at a molecular end or in a molecular chain of one molecule with a polyorganohydrogensiloxane having two or more hydrogen atoms bonded to silicon atoms in one molecule. Such system is used as a sealing agent or a potting agent by utilizing its good weather resistance, water resistance and thermal resistance. However, such system has limited applications because a cost is high, an adhesion property is not good and fungus easily grows. Further, said polyorganosiloxane generally has poor compatibility with an organic polymer. When a polyorganohydrogensiloxane and an organic polymer having an alkenyl group are intended to be cured, hydrolysis and a dehydrogenation condensation reaction of the polyorganohydrogensiloxane are promoted owing to phase separation, and thus sufficient mechanical properties are not achieved because of voids.